It is known that a gear workpiece always has to carry out an additional rotational movement during grinding of helically toothed gears in the hobbing method. This is true as well as in the analog hobbing method in the case of axial relative movement in respect of the grinding and cutting tool, i.e., in the execution of the oscillating feed movements in two-way operations or of the feed and return movements in case of the one-way operation which depends on the module, the number of teeth and the pitch helix angle. This additional rotational movement has to be additively or subtractively superimposed on the rotating movement of the gear workpiece as it would correspond to a straight toothed gear, dependent upon the direction of the relative longitudinal feed movement between the workpiece and the tool.
As shown in German Patent No. 1 248 964, an electronic hobbing machine control has been proposed, in which pulse sequences are derived from the axle of the hob and the axle of the workpiece. These pulse sequences are dependent upon the rpm. They have a specific relationship to each other for a certain tooth number of the gear to be hobbed, in order to attain a direct transmission, for which purpose either pulse dividers or multipliers may be provided for the pulse sequences. The two pulse sequences are compared to each other and a control signal is produced from the comparison for the readjustment of the workpiece drive, so that a positive drive of the workpiece drive is present, in dependence on the hob drive. In order to attain the additional rotational motion that is required for a helical gear workpiece, additional pulses are produced that are supplied to the one pulse sequence, either additively or subtractively.
The additional pulses can, for instance, be derived from the feed shaft for the milling carriage. Such pulse addition or subtraction corresponds, in its effect, to that of a known mechanical differential in hobbing machines for the consideration of the pitch helix angle and which is therefore often called an electrical or electronic differential.
German Patent No. 22 55 514 discloses a gear grinding machine with an electrically controlled workpiece spindle drive and which uses the above mentioned method for the production of additional pulses for the obtention of the additional rotational movement for the helically toothed workpiece. In this machine, the determination and superimposition of the additional rotational movement takes place in a mechanical-electronic manner by measuring the distance of the workpiece carriage carrying out the feed movement in an indirect manner, i.e., by way of a ball rollspindle unit in which the ball rollspindle is coupled to a rotary shaft encoder. The pulses produced are supplied by way of a frequency divider circuit to a coordination circuit which contains a digital-analog converter, which also receives the rpm-dependent pulses of the workpiece, i.e., of the grinding wheel and of the workpiece. By the application of a ball rollspindle unit for the distance measurement (way measurement), one must count, even in the most favorable case, with a transmission error of 0.002 to 0.003 mm, which is too high for tooth flank grinding of highly precise tooth gears. Further imprecisions occur, because in the coordinating circuit the pulse sequence of the workpiece carriage divided in the frequency and the multiplied and divided pulse sequence of the workpiece carriage and the multiplied and divided pulse sequence of the tool are compared directly with the pulse sequence of the workpiece.
Other succession controls that contain a digital travel measuring system with pulse multiplication are also known, such as, for example, an electronic modular system (Zeiss Informations, Oberkochen, No. 80/1972), in which the amplitude fluctuations of the sensed signal show themselves in measurement errors, or, for example, a pulse multiplication with a voltage-controlled oscillator (Roland Best, Theory and application of the phase-lock loop, Elektroniker, No. 10, 1976), in which the output signal is imbued with a drag error. These processes and measuring systems cannot, however, be applied to the tooth flank grinding of highly precise gears because they are either too imprecise or work too slowly.